1. Field of the Invention
This invention relates generally to optical communications networks, and, more particularly, to methodologies and concomitant circuitry for mitigating transient effects in the networks caused by attenuator adjustments which compensate for signal power changes.
2. Description of the Background
Recent research advances in optical Wavelength Division Multiplexing (WDM) technology have fostered the exploratory development of optical networks that are orders of magnitude higher in transmission bandwidth than existing commercial networks. While such an increase in throughput is impressive on its own, a corresponding decrease in network latency can also be achieved in the same networks. Thus, it is clear that the Next Generation Internet (NGI) vision of providing ultra high-speed networks that can meet the requirements for supporting new applications, including national initiatives, is indeed feasible.
However, in both commercial networks and exploratory networks, network reconfigurations, failures, protection switching, and even the fact that not all signals originate at the same point in the optical network (that is, a network element may drop an incoming signal for delivery to a destination device, or may add an incoming signal from a source device onto the optical network) may cause abrupt changes of the power levels of the signals propagating in such optical networks; to fully realize the benefits of the NGI applications, there are potentially deleterious effects to overcome because of such power-changing mechanisms.
First, since network elements contain optical amplifiers, it is known that if the power in a given wavelength serving as the input to an amplifier is large relative to the power in other incoming wavelengths, the dominant wavelength is emitted with more power than the other wavelengths and the power in each of the other wavelengths is reduced. This dominance by the given wavelength causes unequal signal-to-noise ratios for the signals propagated by the wavelengths which, in turn, can cause system degradation. To compensate for such incoming power variations in a conventional arrangement, a servo-controlled attenuator is inserted before each amplifier to serve as a power equalizer. In particular, an optical attenuator is interposed in the path of the incoming signal for each wavelength, and the attenuator's setting is a value that is based upon the history of the optical power that has entered the attenuator. In normal operation, the attenuator settles to an equilibrium state wherein the setting is typically a mid-range value (in the range between a maximum attenuation and a minimum attenuation) based upon desired network operating characteristics, such as the necessary signal-to-noise ratio. To achieve the equilibrium state, the power in the incoming signal is measured and then compared to a "comparison value", which is also selected in view of the network operating characteristics. Then, if the incoming power is too high relative to the comparison value, the attenuation can be increased to offset the high power signal; conversely, the attenuation can be decreased to increase the signal serving as the network element's input. In the extreme case of no measurable input power, the attenuator is set to a mode whereby no attenuation ("no attenuation" mode for later reference) is provided by the attenuator.
Power fluctuations are typically measurable at the input or output of the servo-controlled attenuators. Servo-controlled attenuators exhibit transient settling times before compensating for the power fluctuations and reaching equilibrium; moreover, depending upon their design, such settling times can be long relative to the time constants of other components in the optical network. During the settling time, system performance may be degraded, so an objective in the provision of a power-correcting attenuator network is the minimization of such settling time.
Second, it is also known that the activity of compensating for power fluctuations in a given wavelength by an upstream attenuator impacts on the operation of downstream attenuators and can induce transient settling times in the downstream attenuators. As before, during periods of adjustment, a given wavelength may predominate at a downstream amplifier, and S/N can be degraded. Thus, power fluctuations in an upstream link can cause a "rippling effect" in downstream network elements, and must be mitigated to maintain system performance.
Third, transient conditions caused by power variations of one wavelength channel can even be coupled to other wavelength channels due to the cross-saturation effects of an amplifier; this is especially true if the amplifier is an Erbium-doped fiber amplifier (EDFA) which is not gain-clamped--such EDFAs are typically used in present-day optical networks. This mechanism can be responsible for sustained power fluctuations in large scale optical networks composed of closed loops. Such a network transient response depends upon the magnitude of the initial power perturbation, the speed of the servo-controlled attenuators, the design of the EDFAs, the network topology, and the add/drop characteristics of the network elements, as well as the interactions of the foregoing mechanisms and components.
It is now understood in the art that elimination of coupling between wavelength channels can be achieved by using gain-clamped EDFAs or fast servo-controlled attenuators, that is, attenuators that have response times which are an order of magnitude faster (in the range of 10-100 microseconds) than the corresponding amplifiers (about 1 millisecond).
An article fully discussing the effects of transients induced by the operation of conventional servo-controlled attenuators is published in the Conference Proceedings of the Optical Fiber Communication Conference (OFC) and International Conference on Integrated Optics and Optical Fiber Communication (IOOC), TuR1-1, pgs. 246-248, 1999, and is entitled "Transient Effects in Wavelength Add-Drop Multiplexer Chains".
However, because of the high bit-rate signals in an optical network, even fast-operating attenuators operating in aforementioned speed range will not preclude degraded S/N ratios during the adjustment time, either in a given attenuator or in the downstream attenuators impacted by the transient effects of the given upstream attenuator. The prior art is devoid of teachings or suggestions relating to mitigation of transient oscillations caused by attenuators during periods in which an attenuator is adjusting for shifts in incoming power.